Today so called centrifugal pumps are used to transport liquids by the conversion of rotational kinetic energy to the hydrodynamic energy of the liquid flow. The rotational energy is typically generated by a motor. The pump has a housing, or casing, and an impeller is arranged inside the housing. The fluid enters the impeller along or near to a rotating axis of the impeller and is accelerated by the impeller, flowing radially outward towards an outlet of the housing, from where it exits.
Most centrifugal pumps are not self-priming. Then the pump housing must be filled with liquid before the pump is started, otherwise the pump will not be able to function. If the pump housing becomes filled with gases or vapors, the impeller becomes gas-bound and incapable of pumping the liquid. To ensure that a centrifugal pump remains primed (filled with liquid) and does not become gas-bound, most centrifugal pumps are located below the level of the source from which the pump is to draw the liquid. The same effect can be obtained by supplying liquid to the pump suction side of the pump. This liquid is then supplied under pressure, for example by another pump or by implementing the pump as a so called self-priming, centrifugal pump that recirculates a part of the liquid vie a liquid return conduit.
Self-priming, centrifugal pumps have been described in a number of documents, such as in U.S. Pat. No. 6,585,493 where a self-priming, centrifugal pump has a pump housing with an inlet opening and an outlet piece. An impeller wheel rotates inside the pump housing. The inlet opening is connected with a liquid ring pump section that has an auxiliary housing with an internal pump screw. The pump screw rotates together with the impeller wheel and a recycling (recirculation) pipe for pumped liquid connects the outlet piece with the auxiliary housing. The pump is self-primed by virtue of the recycling pipe that returns a part of the pumped liquid to or near the inlet of the pump during pumping, which means that it is primed during operation even if some gas should be present in the pumped liquid.
The pump screw in the auxiliary housing has a helical blade and is coaxially arranged with the impeller. The auxiliary housing is symmetrical and is arranged with its center axis in parallel and offset to a rotational axis of the pump screw, which enables the pump screw to transport to the impeller any gas that might be present in the liquid.
WO 2009/007075 discloses another self-priming, centrifugal pump that is similar to the one previously described but for a different connection of the recycling pipe, which is connected from the impeller housing to the housing that holds the pump screw.
The prior art is successfully employed as self-priming, centrifugal pumps and are able to pump liquids where some gas or vapor is present. The pump efficiency, i.e. the ratio of the power imparted on the fluid by the pump in relation to the power supplied to drive the pump, is often reasonably good but it is estimated that it may still be improved.